Such logging tools, commonly referred to as being of the "gamma-gamma" type, including a source of gamma rays, and at least one, and often two, gamma ray or photon detectors (e.g. of the NaI type) that are offset longitudinally from the source along the longitudinal axis of the sonde. The gamma rays emitted by the source penetrate into the formation where they are subjected to interactions, and they are detected and counted by the detectors which also measure their energies. The number of gamma particles is shown as a function of detected photon energy, and on the basis of that information, characteristics of the formation such as density and photoelectric absorption factor are calculated. Such tools are described, for example, in European patent application No. 379 813, French patent application No.93 11896 filed on Oct. 6, 1993, and in U.S. Pat. Nos. 4,958,073 and 4,661,700.
Known tools of that type usually include two detectors, namely a near detector and a far detector, which are disposed at distances from the source that are respectively equal to about 15 cm and to about 40 cm. Both detectors are of the attenuation type, i.e. the number of particles detected per unit time is in an inverse relationship with the density of the formation.
Tools are also known that are provided with a "backscattering detector" that has a so-called "non-negative" response to an increase in the density of the formation, unlike the attenuation type near and far detectors (whose response to an increase in density is negative). The backscattering detector is disposed in the immediate proximity of the source, and it is highly collimated.
U.S. Pat. No. 5,282,133 discloses a method and apparatus using a logging tool that includes a backscattering gamma detector near to the source and an attenuation detector far from the source. The data resulting from measurement is initially subjected to preprocessing in order to correct gain and noise errors, then processing is applied firstly to the logarithm of the data from the far detector and secondly to the data from the backscattering detector whereby they are subjected to compression by a main component analysis technique that generates main component vectors which are then subjected to an inverse filter in order to determine the looked-for parameters; the inverse filter is generated using a calibration data base.
In addition, the article by G. L. Moake entitled "A new approach to determining composed density and Pe values with a spectral-density tool", published in the journal S.P.W.L.A., 32nd Annual Logging Symposium, Jun. 17-19, 1991, describes a technique of processing data from a gamma ray tool having two attenuation detectors, one far from the source and the other near to the source. In that known technique, the idea is to minimize, in the least squares sense, the difference between real measurements and data calculated on the basis of a model that is intended to represent the response of the tool to given external conditions.
In general, the determination of density and of photoelectric absorption (Pe) depends on the depth of investigation (in a radial direction) and on resolution; where resolution corresponds substantially to the distance between the source and the detector. The greater the distance between the detector and the source, the greater the investigation depth but the smaller the resolution.
Furthermore, the wall of a borehole presents irregularities of shape and it is also lined with "mudcake" formed by drilling mud caked on the geological formations along the wall. Mudcake influences measurement since the emitted and detected gamma rays or photons are subjected to interactions therein, and this influence increases with increasing thickness of mudcake.
It is therefore important to benefit both from good resolution which is obtained by having a source-to-detector distance that is as short as possible, and from great investigation depth which is obtained by having a source-to-detector distance that is as long as possible. By way of example, the investigation depth lies in the range 5 cm to 12.5 cm. Resolution is mainly a function of the distance between the source and the detector.
Presently known logging tools and methods for determining density and photoelectric absorption present limitations.
It is very difficult with known tools to benefit from resolution of less than about 40 cm even under the best possible measurement conditions, namely: borehole wall of regular shape, good application of the pad, and thin layer of mudcake (less than 1.25 cm).
In other words, measurement compensation to take account of the presence of mudcake is acceptable so long as the thickness of the mudcake is small, typically less than 1.25 cm. Unfortunately, it is not unusual for its thickness to be greater than that. In addition, it is very difficult or even impossible to physically measure the thickness of the mudcake. Thus, if the thickness of the mudcake is greater than the above threshold value, then measurement error can be relatively large and the operator has no way of knowing and thus quantifying the error.
In addition, known apparatuses and methods offer low or medium resolution (about 40 cm) even when the pad is correctly applied against the wall, and when the thickness of the mudcake is small (less than 1.25 cm).